1. Field of the Invention
This invention relates to a method for producing a compound-type superconducting wire such as Nb.sub.3 Sn or V.sub.3 Ga superconducting wire, and more particularly it relates to a method for producing such a compound-type superconducting wire excellent in both electrical and mechanical properties from a sintered metallic mass having good working characteristics.
2. Description of the Prior Art
A superconducting material is generally formed by producing a wire containing a plurality of metals, which are capable of forming a compound having superconducting characteristics through a reaction, and subjecting the wire to a heat treatment to cause the reaction to occur between or among the metals. For example, to obtain a superconducting material of Nb.sub.3 Sn, a composite wire enclosing Nb wires in bronze, which is a Cu-Sn alloy, is produced first. Then, the composite wire is subjected to a heat treatment so as to cause the following reaction between the Nb wires and Sn in the bronze: EQU 3Nb+Sn+Nb.sub.3 Sn,
thereby forming a continuous Nb.sub.3 Sn structure in the matrix of Cu(Bronze process). Or, a Sn plating is applied onto a wire made of a Cu-Nb alloy and the resulting Sn-plated wire is then subjected to a heat treatment, thereby causing the above-described reaction to occur in-situ process). Alternatively, subsequent to mixing Cu powder and Nb powder and sintering the resultant powder mixture, the resulting sintered mass is drawn into a wire and a Sn plating is applied onto the surface of the wire, followed by an application of a heat treatment so as to cause the above-described reaction to occur(Powder process). Of the above processes, the powder process is considered to be superior to the other processes in that, since pure Nb is dispersed in the form of very fine powder in the Cu-matrix, the proportion of Nb can be freely selected, its production process is relatively simple and easy, and the electrical properties of a product to be obtained are good. However, the powder process still involves many technical problems awaiting an improvement or solution.
Namely, in order to produce a Nb.sub.3 Sn-type super-conducting wire from a Cu-Nb green compact in accordance with the conventional powder process, powder of the hydride of Nb obtained by hydrogenating Nb powder in a flow of hydrogen gas is first of all ground further in a disk mill or the like to reduce its mean particle size to about 50 .mu.m or less. The thus-ground powder is heated in vacuo to carry out dehydrogenation annealing, thereby obtaining Nb powder containing oxygen in a proportion of 0.08% by weight or less. Then, the Nb powder and Cu powder are mixed together and formed under pressure by rubber-pressing or the like into a green compact. It is then extruded and drawn into a wire. A Sn plating is then applied onto the surface of the wire. Subsequently, the resulting Sn-plated wire is heated so as to diffusion anneal same and to produce Nb.sub.3 Sn. Such a series of steps is however accompanied by various inconvenience such as those described below:
(1) When heating the powder of the hydride of Nb in vacuo to subject same to dehydrogenation annealing, Nb powder is sintered, thereby to make the Nb powder coarse. Thus, in some instances, it becomes necessary to grind the resulting Nb powder again.
(2) It is required to carry out the dehydrogenation reaction at a relatively low temperature to avoid the occurrence of such sintering. Corollary to this, a longer reaction time is required.
(3) As the dehydrogenated Nb powder is activated at the surface thereof and thus liable to reoxidation, the workability of resulting green compact or sintered mass in subsequent steps would be lowered.
(4) Since Nb powder is extremely fine, its particles having strong tendency to form agglomerates. Thus, in order to obtain a uniform mixture of Cu powder and Nb powder, it is necessary to mix them together in a wet state by means of a ball mill or to adopt some special mixing technique. However, in the course of mixing pure Nb powder and pure Cu powder together, they are susceptible of undergoing a surface oxidation, thereby creating a problem similar to that referred to in the above item (3).
Such problems would become serious obstacles in the subsequent wire drawing step, composition step and/or reaction step between Nb and Sn, leading to deterioration of the electrical and mechanical characteristics of a resulting wire.
In addition, when Nb and Sn are caused to react with each other and a superconducting phase is thus formed by subjecting a composite wire formed of said two metals to a heat treatment, voids, which are generally called Kirkendall voids, occur inevitably in the wire due to an alteration of the atomic arrangement caused by the reaction. Furthermore, as the superconducting compound formed in the wire is not suitable for further working such as rolling or drawing after the production of the superconducting wire through the above heat treatment, it is the present state of the art that the thus-heat treated composite wire is used as a superconducting wire without applying such a subsequent working thereto, while still containing such voids. It is well-known that the presence of such voids in the superconducting wire lowers its thermal conductivity, becomes starting points of breakage of the brittle layer of said compound when stress is exerted onto the wire and also becomes a cause for deteriorated electrical properties of the wire.
However, a superconducting compound is too brittle to apply a conventionally-employed void elimination technique such as rolling or drawing after the formation thereof. Therefore, the presence of such voids is a very serious problem for compound-type superconducting wires. The present assignee has already proposed in Japanese Patent Application No. 88053/1980 filed June 27, 1980 an effective method for the elimination of such voids, in which a superconducting wire obtained through a heat treatment is held for a certain period of time in a gaseous atmosphere of a temperature and pressure sufficiently high to cause the wire to undergo plastic deformation, thereby collapsing said voids, diffusion bonding the thus-collapsed portions and producing a super-conducting wire of high density.